The invention relates to coated particles and particularly to coated phosphor particles. More particularly the invention is concerned with coated electroluminescent phosphor particles.
Finely divided materials are used in a great many products and applications. However, it is sometimes the case that the surface properties of the particle are not completely satisfactory for the intended application. In such cases, various surface modification techniques may be used to improve the performance of the particle. One way of changing the surface properties of a particle material is to coat each particle with a substance whose properties are both substantially different from those of the base material and more compatible with the environment of the intended application. For instance, phosphor particles may be protected from the harmful effects of the mercury plasma in an operating fluorescent lamp by the deposition of fully encapsulating coatings (U.S. Pat. Nos. 4,585,673; 5,051,277). In a similar way, electroluminescent phosphor may be protected from the effects of moisture in an operating electroluminescent lamp by the application of protective coatings (U.S. Pat. Nos. 5,080,928; 6,064,150).
In the cited references, as well as in other similar references, the coatings are deposited via chemical vapor deposition reactions carried out in fluidized particle beds at elevated temperatures. The vaporized coating precursors are brought into the heated fluidized bed via tubing heated so that the vaporized precursors do not condense or react prior to entering the bed and adsorbing on the fluidized particle surfaces. To accomplish this, the reaction temperature, that is, the fluidized bed temperature, generally must be no lower than the lowest heated precursor transport line temperature. This is because a fluidized bed is an excellent heat exchange medium. It is practically impossible to maintain the temperature of a gas delivery line entering a fluidized bed at a temperature substantially higher or, lower than the temperature of the fluidized bed itself. The gas delivery line then cannot have a temperature substantially different from the temperature of the fluidized particles. Therefore, such equipment cannot be used with a coating deposition processes when the maximum deposition temperature is less than the minimum heated delivery line temperature. The coating material would otherwise react in, coat and eventually clog the delivery line.
One class of coating deposition processes which cannot be carried out using a traditional fluidized bed deposition system are the so called vapor deposition processes (A. Kubono, et al, Prog. Polym. Sci., 19, 389-438, 1994). Applying such a process to a typical fluidized bed reactor, the coating material to be deposited would be transported into the fluidized bed reactor as a vapor via an inert carrier gas, at which point the vaporized coating material would mix with the finely divided coating substrate. However, to adsorb (deposit) the vaporized coating material on the surfaces of the fluidizing particles, the fluidized bed temperature must be substantially lower than that of the incoming gas stream. This, however, is physically impossible since the excellent heat exchange properties of the fluidized bed causes the temperature of the incoming gas line near to the fluidized bed to be substantially the same as that of the fluidized bed itself. Thus, the coating material either deposits in the end of the gas delivery tube (before coming into contact with the fluidized particles) or it is not deposited at all, depending on the temperature of the fluidized bed and the partial pressure of the vaporized coating material in the incoming gas stream.
A second class of coating deposition processes which cannot be carried out using a traditional fluidized bed deposition systems are the so called vapor deposition polymerization processes (A. Kubono, et al, Prog. Polym. Sci., 19, 389-438, 1994). Processes of this type are similar to the simpler vapor deposition processes described above, except that the coatings are built up by the repeated reaction (polymerization) of the vaporized precursor molecules on the surfaces of the fluidized particles, rather than by simple adsorption. Such coatings may be formed by the reaction of a single precursor molecule, for example, by the polymerization of p-xylylene (P. Kramer, et al, J. of Polymer Science: Polymer Chemistry Edition, 22, 475-491, 1984.), or by the reaction of two different monomeric precursor molecules, for example by, the reaction of a diamine with a dianhydride to form a polyimide (K. Iida, et al, Japanese Journal of Applied Physics, 28, 2552-2555, 1989). Otherwise, the considerations are identical to those outlined above for the simpler situation involving vapor deposition only.
Accordingly, there is a need for a method and equipment the use of which useful coatings can be uniformly deposited on the surfaces of fluidized particulate materials by vapor deposition processes at temperatures lower than those of the heated coating precursor transport lines.
A new fluidized bed particle coating method by the use of which coatings can be uniformly and conveniently deposited on the surfaces of fluidized particulate materials by vapor deposition processes at temperatures lower than those of the heated coating precursor transport lines. Particle materials with relatively low surface temperatures may be brought into close proximity with a coating precursor containing gas stream characterized by a substantially higher gas volume temperature in such a way that the precursor molecules adsorb or condense on the relatively cold particle surfaces without also condensing on the equipment surfaces. Further, if the adsorbed precursor molecules are capable of reacting or polymerizing on the relatively cold particle surfaces, thus forming substantially continuous coatings on those surfaces, then they may do so without significant deposition of the coating material on the equipment surfaces.